Heart muscle, like all aerobic tissues, obtains the energy which is required for contraction by using the available energy of respiration to synthesize ATP in the process of oxidative phosphorylation. This energy -linked respiration is catalyzed by the electron transport chain of the mitochondria and is localized in the inner membrane of the mitochondria. The cytochrome b-c1 complex is a lipoprotein segment of the inner mitochondrial membrane, and electron transfer through this complex generates the free energy for synthesis of ATP at the second "coupling site." The long-range objective of this investigation is to elucidate the mechanism of electron transfer through the cytochrome b-c1 complex and establish how this electron transfer is coupled to synthesis of ATP. Particlar emphasis will be placed on investigating selected aspects of the structure of cytochrome c1 as it exists within the membrane and understanding how cytochrome c1 regulates electron transfer through the cytochrome b-c1 complex. Experimentation will be extended to identify currently unrecognized components of the cytochrome b-c1 complex and assign to these a structural and/or functional role in the electron transfer and energy conserving activities of this segment of the mitochondrial respiratory chain.